Silicon-based visible and near-infrared optoelectric devices

ABSTRACT

In one aspect, the present invention provides a silicon photodetector having a surface layer that is doped with sulfur inclusions with an average concentration in a range of about 0.5 atom percent to about 1.5 atom percent. The surface layer forms a diode junction with an underlying portion of the substrate. A plurality of electrical contacts allow application of a reverse bias voltage to the junction in order to facilitate generation of an electrical signal, e.g., a photocurrent, in response to irradiation of the surface layer. The photodetector exhibits a responsivity greater than about 1 A/W for incident wavelengths in a range of about 250 nm to about 1050 nm, and a responsivity greater than about 0.1 A/W for longer wavelengths, e.g., up to about 3.5 microns.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/267,618, filed Oct. 6, 2011, which is a continuation of U.S.patent application Ser. No. 12/776,694, filed May 10, 2010 (now U.S.Pat. No. 8,080,467), which is a continuation of U.S. patent applicationSer. No. 12/365,492, filed Feb. 4, 2009 (now U.S. Pat. No. 7,781,856),which is a continuation of U.S. patent application Ser. No. 11/445,900,filed Jun. 2, 2006 (now U.S. Pat. No. 7,057,256), which is acontinuation of U.S. patent application Ser. No. 10/950,230, filed Sep.24, 2004, now U.S. Pat. No. 7,057,256, which is a continuation-in-part(CIP) of U.S. patent application Ser. No. 10/155,429, filed May 24, 2002(now U.S. Pat. No. 7,390,689) all of which are herein incorporated byreference in their entirety.

FEDERALLY SPONSORED RESEARCH

The invention was made with Government support under contractDE-FC36-016011051 awarded by Department of Energy (DOE). The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to silicon photodetectors, andmore particularly to silicon photodetectors that are suitable fordetecting electromagnetic radiation over a wide wavelength range, e.g.,from visible to the infrared, with enhanced responsivity.

Modern semiconductor circuits are predominantly based on silicon, whichcan be readily procured at a lower cost than any other semiconductor andcan be easily oxidized. Further, a band gap of 1.05 eV renders siliconsuitable for detection of visible light and conversion of sunlight intoelectricity. Silicon, however, has several shortcomings. For example, itis an indirect band-gap material, and hence it is a relatively poorlight emitter. In addition, silicon is not particularly suitable for usein detecting radiation having long wavelengths, such as, infraredradiation employed for telecommunications. Although other semiconductormaterials are available that can detect long wavelength radiation betterthan silicon, they are generally more costly and can not be readilyintegrated in optoelectronic circuits that are primarily silicon-based.

Hence, there is a need for silicon-based photodetectors with enhancedradiation-absorbing properties, particularly in wavelengths beyond theband-gap of crystalline silicon. There is also a need for suchphotodetectors that exhibit enhanced responsivity in detecting radiationover a wide wavelength range. Further, there is a need for methods ofmanufacturing such silicon-based photodetectors.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a photodetector thatincludes a silicon substrate having a surface layer, configured forexposure to external radiation, that is doped with sulfur inclusionswith an average concentration in a range of about 0.5 atom percent toabout 5 atom percent so as to exhibit a diodic current-voltagecharacteristic. The photodetector farther includes a plurality ofelectric contacts, e.g., in the form of metallic coatings, that aredisposed on selected portions of the substrate for applying a selectedreverse bias voltage, e.g., in a range of about 0.1 to about 15 volts,to the surface layer for facilitating generation of an electricalsignal, e.g., a photocurrent, in response to exposure of the surfacelayer to electromagnetic radiation. The surface layer is configured suchthat an electrical signal in response to an incident radiation having atleast a wavelength in a range of about 250 nm to about 1050 nm can begenerated at a responsivity greater than about 1 A/W. Preferably, theresponsivity over the entire range of about 250 nm to about 1050 nm isgreater than about 1 A/W. For example, the responsivity to at least onewavelength in this wavelength span, and preferably to all wavelengths inthis span, can be in a range of about 1 A/W to about 100 A/W, or in arange of about 10 A/W to about 100 A/W, or in a range of about 10 A/W toabout 200 A/W.

In another aspect, the photodetector can exhibit an average responsivity(i.e., responsivity averaged over a wavelength range) to wavelengths ina range of about 250 nm to about 1050 nm that is greater than about 1A/W, e.g., an average responsivity in a range of about 1 A/W to about100 A/W, or an average responsivity in a range of about 10 A/W to about200 A/W.

In another aspect, the photodetector can exhibit a responsivity greaterthan about 1 A/W (e.g., in a range of about 1 A/W to about 100 A/W) forwavelengths in a range of about 250 nm to about 600 nm, as well as in arange of about 600 nm to about 1050 nm.

In a related aspect, the photodetector can exhibit a responsivitygreater than about 0.1 A/W for incident radiation having at least onewavelength in a range of about 1050 nm to about 2000 nm, and preferablyin a range of about 1050 nm to about 3500 nm. Preferably, thephotodetectors responsivity to all wavelengths in this range (about 1050nm to about 3500 nm) is greater than about 0.1 A/W. For example, thedetector's responsivity in this wavelength span can be in a range ofabout 0.1 A/W to about 100 A/W.

In another aspect, the photodetector can exhibit an average responsivitygreater than about 0.1 A/W (e.g., a responsivity in a range of about 0.1A/W to about 100 A/W) for radiation having wavelengths in a range ofabout 1.050 nm to about 2000 nm, and preferably in a range of about 1050nm to about 3500 nm.

Photodetectors according to the teachings of the invention, such asthose described above, provide a marked improvement over conventionalsilicon photodiodes where responsivities greater than 1 A/W forwavelengths in a range of about 250 nm to about 1050 nm are unknown.Further, responsivities of conventional silicon photodiodes degradedrastically for wavelengths beyond 1050 nm, and are significantly lessthan those that can be achieved by photodetectors of the presentinvention.

In another aspect, the surface layer can have a thickness in a range ofabout 10 nanometers to about 1 micron, and a microstructured morphology.For example, the microstructured layer can be formed by a plurality ofconical, microstructures, each of which can be composed of a coresilicon portion forming a junction with a surface portion that is dopedwith sulfur. A term “conical structure or microstructure” as used hereinrefers to a generally cone-like or columnar protrusion above the siliconsurface that can have vertical walls, or slanted walls that taper in thevertical direction. The microstructures can have a height in a range ofabout 0.5 micron to about 30 microns protruding from a base portion onthe surface of the silicon substrate to a tip portion having a radius ofcurvature of about a few hundred nanometers. The gradient of sulfurconcentration across the junction is substantially abrupt, although insome cases it can be gradual with the core portion having some sulfurdopants. The sulfur-doped surface layer can comprise siliconnanocrystals having diameters in a range of about 10 about 50nanometers.

In another aspect, the invention provides a photodetector that includesa silicon substrate having a microstructured layer incorporatinginclusions containing an electron-donating constituent. The microstructured layer is adjacent an underlying bulk silicon portion andforms a diode junction therewith. The term “diode junction.” is known inthe art and generally refers to a junction that can exhibit currentrectification (e.g., a junction that exhibits drastically differentconductivities in one bias direction relative to the other). Awell-known example of a diode junction is a p-n junction. An electricalcontact, is disposed on a surface portion of the microstructured layersuch that at least another surface portion of that layer remainsavailable for receiving incident electromagnetic radiation. Thephotodetector can further include another electrical contact disposed ona surface of the bulk silicon portion opposed to the microstructuredlayer. The electrical contact layers allow application of a reverse biasvoltage to the substrate to facilitate generation of an electricalsignal, e.g., a photocurrent, in response to exposure of themicrostructured layer to incident radiation having wavelengths in rangeof about 250 nm to about 1050 nm at a responsivity in a range of about 1A/W to about 200 A/W.

In a related aspect, the substrate can generate a photocurrent inresponse to irradiation of the microstructured layer with radiationhaving wavelengths in a range of about 1050 nm to about 3500 nm at aresponsivity in a range of about 0.1 A/W to about 100 A/W.

In another aspect, the electron-donating constituent can be, forexample, sulfur, chlorine or nitrogen, and can be present in themicrostructured layer at a concentration in a range of about 0.5 toabout 5 atom percent, or in a range of about 0.5 to about 1.5 atompercent, or in a range of about 0.5 to about 1 atom percent.

In another aspect, the present invention provides a photodetector foruse in the visible and the infrared regions of the electromagneticspectrum that includes a crystalline silicon substrate having amicrostructured surface layer characterized by a plurality ofsubstantially conical microstructures. The microstructured layerincludes a doped layer having a plurality of silicon nanocrystals thatcontain an electron-donating dopant, such as sulfur, chlorine ornitrogen, with an average concentration in a range of about 0.5 to about1.5 atom percent. The microstructured layer exhibits a diodiccurrent-voltage curve and is configured to receive an externalradiation. A plurality of electrical contacts are disposed on selectedportions of the substrate for applying a reverse bias voltage to themicrostructured layer in order to facilitate generation of an electricalsignal in response to irradiation of at least a portion of that layer.The photodetector exhibits a responsivity greater that about 1amperes/watt (A/W) for radiation wavelengths in a range of about 250 nmto about 1050 nm and a responsivity greater than about 0.1 amperes/wattfor radiation wavelengths in a range of about 1050 nm to about 3500 nm.

Further understanding of the invention can be obtained by reference tothe following detailed description in conjunction with the associateddrawings, which are briefly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart depicting various steps in an exemplaryembodiment for generating a microstructured silicon wafer suitable forabsorbing radiation,

FIG. 2 is a schematic diagram of an exemplary apparatus formicrostructured silicon wafers in accordance with one aspect of theinvention,

FIG. 3 is a scanning electron micrograph of a silicon surface, obtainedat a 45-degree angle relative to the surface, after irradiation of thesurface with 100 fsec pulses having a central wavelength of 800 nm and afluence of about 5 kJ/m²,

FIG. 4 schematically illustrates an experimental set-up for performingresistivity and Hall effect measurements on a silicon samplemicrostructured in accordance with the teachings of the invention,

FIG. 5 presents a plurality of graphs depicting wavelength absorptanceof prototype microstructured silicon wafers as a function of an averagenumber of 100 laser shots (8 kJ/m²) per location employed formicrostructuring the wafers in the presence of SF₆,

FIG. 6 presents a plurality of graphs depicting wavelength dependence ofabsorptance of a plurality of prototype silicon waters annealed atdifferent temperatures subsequent to their microstructuring by aplurality of femtosecond laser pulses (the room temperature (300 K) datacorresponds to a wafer that was not annealed),

FIG. 7 presents graphs depicting wavelength dependence of absorptancefor exemplary femtosecond-laser-formed and nanosecond-laser-formedmicrostructured silicon waters, before and after thermal annealing at875 K for about 30 minutes (the absorptance of an unstructured siliconsubstrate as a function of wavelength is also shown for comparativepurposes),

FIG. 8A presents graphs depicting absorptance as a function, ofwavelength for a plurality of prototype silicon wafers microstructuredby exposure to 500 laser shots of femtosecond, pulses (fluence of 8kJ/m²) in the presence of different gases in the absence of annealing(the graph designated vacuum relates to wafer microstructured in avacuum chamber at a base pressure less than about 10⁻⁶ bar),

FIG. 8B presents absorptance data as a function of wavelength for aplurality of prototype silicon wafers microstructured by exposure to 500shots of 100 femtosecond laser pulses (fluence of 8 kJ/m² in thepresence of sulfur-bearing and non-sulfur-bearing gases) indicating thepositive effects of sulfur incorporation in enhancing below band-gapabsorptance,

FIG. 9 presents graphs depicting wavelength dependence of absorptancefor a plurality of prototype silicon wafers microstructured byfemtosecond laser pulses in the presence of SF₆ at different partialpressures,

FIG. 10 schematically illustrates a p-n junction in thermal equilibrium,

FIG. 11A is a perspective view of a photodetector formed in accordancewith one embodiment of the invention,

FIG. 11B is a side view of the photodetector depicted in FIG. 11A,

FIG. 12 presents graphs depicting current-voltage characteristics of aplurality of exemplary silicon wafer microstructured by exposure tofemtosecond laser pulses in the absence of annealing as well as in thepresence of annealing at different annealing temperatures,

FIG. 13 presents graphs depicting responsivity of a plurality of siliconwafers microstructured by exposure to femtosecond laser pulses in thepresence of SF₆ (with no annealing and with annealing at differenttemperatures) as a function of wavelength in comparison with, that of acommercial photodiode illustrating that a significantly enhancedresponsivity can be obtained by judicious annealing of wafers subjected,to laser microstructuring,

FIG. 14 present graphs depicting current-voltage characteristics of aplurality of exemplary prototype silicon wafers microstructured byfemtosecond laser pulses at different laser fluences and each annealedat 825 K,

FIG. 15A presents comparative responsivity measurements for prototypesilicon wafers microstructured by femtosecond laser pulses at different,fluences and each annealed at 825 K for about 30 minutes,

FIG. 15B presents comparative responsivity measurements for prototypesilicon wafers microstructured by femtosecond laser pulses at differentfluences and each annealed at 1075 K for about 30 minutes,

FIG. 16A presents current-voltage characteristics for n-doped andp-doped silicon substrates microstructured by exposure to femtosecondlaser pulses at a fluence of 6 kJ/m² followed by annealing at 825 K forabout 30 minutes,

FIG. 16B presents current-voltage characteristics for n-doped andp-doped silicon substrates microstructured by exposure to femtosecondlaser pluses at a fluence of 4 kJ/m² followed by annealing at 1075 K forabout 30 minutes.

FIG. 17A presents responsivity measurements for prototype exemplaryn-doped and p-doped silicon substrates microstructured at two differentlaser fluences (4 kJ/m² and 6 kJ/m²) and annealed at 825 K for about 30minutes,

FIG. 17B presents responsivity measurements for prototype exemplaryn-doped and p-doped silicon substrates microstructured by femtosecondlaser poises at a fluence of 4 kJ/m² and annealed at 1075 K for about 30minutes,

FIG. 18 presents a graph depicting responsivity as a function of backbias for an n-doped silicon substrate microstructured via irradiation byfemtosecond laser pulses at a fluence of 4 kJ/m² and annealed at 825 Kfor about 30 minutes (the measurement was performed by employing a whitelight source with an approximate power of 50 microwatts),

FIG. 19A presents graphs depicting current-voltage characteristics of ann-doped silicon sample microstructured by femtosecond laser pulses at afluence of 4 kJ/m² and annealed at 825 K as a function of operatingtemperature,

FIG. 19B presents a graph depicting the responsivity of amicrostructured silicon wafer to incident radiation having a wavelengthof 1064 nm as a function of operating temperature,

FIG. 20 illustrates current-voltage characteristics for a highresistivity (resistivity greater than 1 Ohm-m) microstructured siliconwafer that can be utilized as a solar cell, with and without one sunillumination,

FIG. 21 illustrates current-voltage characteristics for a lowresistivity (e.g., a resistivity in a range of about 0.01 to about 0.1Ohm-m) microstructured silicon wafer that can be utilized as solar, withand without one sun illumination,

FIG. 22 schematically depicts an exemplary experimental set-up formeasuring field emission of a silicon substrate microstructured inaccordance with the teachings of the invention, and

FIG. 23 shows a graph depicting field emission current generated by asilicon wafer microstructured by exposure to femtosecond laser pulses inthe presence of SF₆ as a function of a potential difference appliedthereto.

DETAILED DESCRIPTION

With reference to a flow chart 10 of FIG. 1, in one aspect, the presentinvention provides a method of fabricating a radiation-absorbingsemiconductor structure in which, in an initial step 10 a, each of aplurality of locations on a surface of a silicon substrate is irradiatedwith one or more laser pulses while exposing the surface to a substancehaving an election-donating constituent so as to generate surfaceinclusions containing a concentration of the electron-donatingconstituent. The laser pulses can have a central wavelength in a rangeof about 200 nm to about 1200 nm, and a pulse width in a range of abouttens of femtoseconds to about hundreds of nanometers. Preferably, thelaser pulse widths are in a range of about 50 femtoseconds to about 50picoseconds. More preferably, the laser pulse widths are in the range ofabout 50 to 500 femtoseconds. The number of laser pulses irradiating thesilicon surface can be in a range of about 2 to about 2000, and morepreferably, in a range of about 20 to about 500. Further, the repetitionrate of the pulses can be selected to be in a range of about 1 kHz toabout 50 MHz, or in a range of about 1 kHz to about 1 MHz. Moreover, thefluence of each laser pulse can be in a range of about 1 kJ/m² to about12 kJ/m², and more preferably in a range of about 3 kJ/m² to about 8kJ/m².

While in some embodiments, the electron donating constituent is a fluid(gas or liquid), in other embodiments it can be a solid deposited overthe silicon surface exposed to laser pulses. By way of example, in someembodiments, the silicon surface can be exposed to a sulfur-containinggas (e.g., SF₆ or H₂S) while irradiated with the laser pulses, in someother embodiments, Cl₂ or N₂ is employed as the electron-donatingsubstance with which the silicon surface is in contact during laserirradiation. In yet other embodiments, selenium, or tellurium can beemployed. Without any loss of generality, in the following discussion,it is assumed that SF₆ is employed as the substance containing anelectron-donating constituent, and that the irradiating pulses havefemtosecond pulse widths, unless otherwise indicated. However, it shouldbe clear to those having ordinary skill in the art that variousembodiments of the invention described below can also be practiced withother fluids or solids and laser pulses having other pulse widths, suchas those listed above.

In some embodiments, the silicon substrate is p-doped (e.g., doped withBoron) with a doping level in a range of about 10¹¹ to about 10¹⁸. Inother embodiments, the silicon substrate is slightly n-doped (e.g.,doped with Phosphorus) with a doping level in a range of about 10¹¹ toabout 10¹⁸, and preferably in a range of about 10¹² to about 10¹⁵. Thesubstrate thickness is not typically a critical parameter, and can be ina range of about 2 microns to about 1000 microns. The substrate can havean electrical resistivity in a range of about 0.001 ohm-m to about 10ohm-m.

FIG. 2 schematically depicts an exemplary apparatus 12 that can beutilized for irradiating a silicon sample with a plurality offemtosecond pulses. The apparatus 12 includes a sample processingchamber 14 that comprises a stainless steel cube 16 with Conflat flangeconnections on each of its six sides. A three-axis, precision motioncontroller 18 is attached to the back side of the cube. This controllercontains two orthogonal micrometer precision axes driven by computercontrolled motors. The third axis is hand-controlled with about 1 milprecision. The controller can translate a one-inch diameter stainlesssteel rod 20 that supports a 2-inch diameter mounting magnet 22 in thecenter of the cube to which a magnetizable sample holder can be mounted.A two-stage roughing pump 24 is coupled to the cube to evacuate thechamber to a base pressure of approximately 10⁻³ torr. Two pressuregauges 26 and 28 are employed to monitor the chamber's pressure. A leakvalve 30 and a gas-handling manifold (not shown) allow introduction ofgases of interest into the chamber. An optical grade, 4.5 inch quartzwindow 32 is attached to the front of the chamber for providing laseraccess. Further, a quick-access viewport door 34 allows rapid loadingand removal of samples as well as safe viewing of the sample duringmicrostructuring.

In this exemplary apparatus, a regeneratively amplified, femtosecondTi:Sapphire laser system (not shown) provides approximately 100 fs laserpulses having a central wavelength of 800 nm at a repetition rate of 1kHz. The laser pulses are focused by a anti-reflection coated,plano-convex lens 36 (e.g., having a local length of about 250-nm)through the quartz window onto the sample surface. In this exemplaryembodiment, the lens is mounted on a single axis linear translationstage and positioned such that its focal point is behind the sample. Bymoving the lens, and hence its focal point, the laser spot size can bevaried at the sample surface (e.g., from a diameter of about 30 micronsto 250 microns). In order to measure the laser spot size, a CCD (chargecoupled device) camera 38 can be placed at the same distance as thesample surface from the lens. A flipper mounted mirror 40 can redirectthe laser beam onto the camera to determine its spot size on the samplesurface. A second CCD camera 42 can measure reflections of lightgenerated by a white light fiber lamp 44 from the sample surface tomonitor progress during microstructuring.

In one exemplary mode of operation, samples attached to a magnetizablesample holder can be loaded through the access door and attached to themounting magnet. Samples can be then positioned in the center of themagnet to allow for maximum translation. The chamber can be evacuated toa base pressure of 10⁻³ torr and then filled to the desired pressurewith an ambient gas (e.g., 0.67 bar of sulfur hexafluoride). The samplecan then be irradiated with a 1-kHz train of 100-femtosecond 800-nmlaser pulses. The fluence of each pulse can be set by selecting a spotsize (e.g., 150 microns) and employing a waveplate/polarizing cubecombination to vary the pulse energy. Either a single spot can beirradiated, or more typically the sample can be translated by utilizingthe motion controller relative to the laser beam. For example, thesample can be translated in a raster scan pattern. An average number ofpulses irradiating a given spot on the sample can be controlled byutilizing a shutter and varying the horizontal translation speed.

The irradiation of the silicon surface with the laser pulses in thepresence of SF₆ at a selected partial pressure, for example, a partialpressure in a range of about 0.005 bar to about 1 bar, can causeformation of a sulfur-rich layer having a sulfur concentration in arange of about 0.1 atom percent to about 5 atom percent and morepreferably, in a range of about 0.5 atom percent to about 1 atompercent. This sulfur-rich layer exhibits an undulating surfacemorphology (topography) with micron-sized surface height variations.Such a textured surface obtained by irradiating a silicon surface with aplurality of temporally short laser pulses in the presence SF₆ (or othersubstances having an electron-donating constituent) is herein, referredto as a micro-structured surface. By way of example, FIG. 3 shows ascanning electron micrograph (SEM) of a such a micro-structured siliconsurface (obtained in a direction forming a 45 degree angle relative tothe surface) after irradiation of the silicon surface with an average of5 shots of femtosecond laser pluses (100 femtoseconds) at a wavelengthof 800 nm and having a fluence of 8 kJ/m² in 0.67 bar of SF₆.

The sulfur-rich layer can be a substantially disordered layer that isseveral hundred nanometers thick and made up of nanocrystallites (e.g.,about 10-50 nm in diameter) and nanopores. Selected area diffraction(SAD) measurements indicate that the surface layer retains a substantialcrystalline order with a strong possibility that amorphous-like materialis present between the nanocrystalline grains. Further, SAD measurementsperformed on silicon substrates exposed to either 10 or 500 pulses perlocation indicate that the substrate below the disordered layercomprises substantially undisturbed crystalline silicon (when acrystalline silicon substrate is utilized). Further, Rutherfordbackscattering spectroscopy (RBS) and energy dispersive X-ray (EDX)emission spectroscopy performed on such exemplary substrates show alarge concentration of sulfur is present in the disordered layer (about1 atom percent). Further, RBS spectra obtained from silicon samplesexposed to different femtosecond (100 femtoseconds) laser shots perirradiation location show that the concentration of sulfur increaseswith increasing laser pulse numbers up to about 50 pulses. Inparticular, the measured sulfur concentration increases from about 0.2atom percent for a sample exposed to 2 pulses per location to about 0.7atom percent for a sample exposed to 50 laser pulses per location withthe sulfur concentration exhibiting a plateau for laser pulses higherthan about 50. It should be, however, understood that the aboveexperimental results are provided simply for further elucidation ofsalient features of the invention, and not as limiting the scope of theinvention. For example, higher sulfur concentrations in the disorderedlayer than those described above can also be obtained, and higher lasershots per location can be utilized.

A number of factors can affect the morphology of the microstructuredsurface. For example, samples microstructured by irradiating a portionthereof with laser pulses, without moving them relative to the laserbeam (i.e., single spot irradiation), exhibit a subtle difference intheir morphology relative to samples microstructured by irradiating aplurality of their surface locations via translation, relative to thelaser beam. With the rest of the experimental parameters left the same,there are two visible differences between translated and stationarysamples. The microstructures in the stationary samples have amicrometer-sized sphere at the tip while the translated samples aresharper and lack the spheres. Further, the translated samples have alarger amount of nanoscale particles spread across the microstructuredsurface.

In addition, the wavelength of the irradiating laser pulses, theirfluence, and pulsewidths can also affect the morphology of themicrostructured surface. For example, in a number of prototype samplesmicrostructured by femtosecond pulses, with increasing the fluence (atconstant shot number) the morphology showed a transition fromlaser-induced periodic surface structures, to a coarsened surface, tosharp microstructures, such as those depicted in the above FIG. 3. Asthe laser pulse fluence increases, there is typically an increase inboth the microstructure height and distance between the microstructures.In general, the fluence is preferably selected to be above a thresholdfluence that would cause melting (e.g., 1.5 kJ/m²). At very highfluences (e.g., greater than 12 kJ/m²), material removal can becomeextreme and gaussian-shaped holes, rather than conical structures, canbe formed in the surface.

Although in many embodiments described herein, femtosecond laser pulsesare employed, in other embodiments, the microstructuring of the surfacecan also be achieved by employing picosecond or nanosecond pulses. Forexample, a number of silicon samples were microstructured by utilizing atrain of 248-nm, 30-ns laser pulses with a flat-fop spatial profile anda fluence of 30 kJ/m² generated by a KrF+ excimer laser in the presenceof 1 bar of SF₆ (an average of 1500 pulses per spot were employed). Thenanosecond-laser-formed microstructures showed some similarities andsome differences relative to the femtosecond-laser-formedmicrostructures. In both cases, the microstructures were roughlyconical, but the structures formed with the femtosecond laser pulseswere smaller and more densely packed. For example, the femtosecondstructures were roughly 8 microns tall and separated by about 4 micronswhile the nanosecond-formed structures were roughly 40 microns tall andseparated by about 20 microns. Further, the femtosecond-formedstructures were covered with nanoparticles 10-50 nm in diameter whilethe surface of the nanosecond-formed structures were much smoother.

Sample silicon wafers exposed, to picosecond pulses (e.g., 10picosecond) in the presence of SF₆ also showed a microstructuredsurface. The average separation between the microstructures exhibited adecrease from about 100 femtosecond pulse widths to about 5 picosecondpulse widths but then showed an increase for pulse widths longer thanabout 5 picoseconds.

The laser wavelength ears also affect the final morphology of themicrostructures. By way of example, a prototype silicon samplemicrostructured with a train of 400 nm pulses exhibited a higher densityof microstructures (but smaller) than those generated by 800 nm pulsesat a similar fluence.

Other factors that can affect the microstructures' morphology includelaser polarization and laser propagation direction relative to theirradiated silicon surface. For example, the direction of microstructuregrowth is parallel to the direction of incident light, and appears to beindependent of the substrate's crystallographic planes. The base ofmicrostructure can have an elliptical shape with the long axisperpendicular to the polarization of the irradiating laser light Withoutbeing limited to any theory, this elliptical shape can be understood asbeing due to a higher absorption of p-polarized light relative tos-polarized light. For circularly polarized light, there is nopreferential ablation, and the base of the microstructures can besubstantially circular.

The fluid or solid in contact with the silicon surface during itsmicrostructuring can also affect the resultant surface morphology. Forexample, utilizing SF₆ or Cl₂ as the ambient gas can lead tosharp-tipped conical structures with a radius of curvature of about 500nm. In contrast, structures made in air, N₂ or vacuum are much morerounded (the radius of curvature of their tips is approximately 2-3microns) than those made in these halogen-containing gases. In a numberof exemplary samples microstructures followed the presence of differentgases, those made in SF₆ exhibited the greatest density ofmicrostructures followed by those made in Cl₂. The microstructuredensities produced in the presence of N₂ were approximately equal tothose made in air, but approximately a factor of two less than thedensities obtained by employing SF₆.

In some embodiments, the laser microstructuring of a silicon wafer isperformed in the presence of a mixture of two or more substances. Forexample, samples microstructured in the presence of a mixture of SF₆ andCl₂ exhibited an increase in the microstructure density at higherpartial pressure of SF₆.

The sulfur-rich microstructured layer has unique optical and electricalproperties. For example, it can exhibit a sheet charge carrier densityin a range of about 10¹² cm to about 10¹⁴ cm². By way of example andonly for illustrative purposes, the results of some resistivity and Halleffect measurements performed on a number of exemplary microstructuredsamples, each formed in an n-doped silicon substrate in accordance withthe teachings of the invention, such as those described above, arepresented below. For resistivity and Hail effect measurements, the vander Pauw technique, well-known to those having ordinary skill in theart, was employed. Briefly, four ohmic electrical contacts were formedon the disordered microstructured layer. The electrical contacts wereformed by initially dipping a microstructured sample in a 5% HF solutionfor 5 minutes to remove any native oxide layer. Subsequently, with amask covering all but the very corners of a selected microstructuredsurface area (e.g., a 10×10 mm² surface area), chromium/gold (Cr/gold)was evaporated onto the surface to form metal contacts at the exposedcorners. A dicing saw was then used to cut slivers (e.g., 0.25 mmslivers) from each side. Cutting the sample edges ensures that thecontacts are only connected to the surface layer and not the substratelayer. Finally, a wire bonder is utilized to connect the contacts on thecorners of the sample to four Cr/Au contact pads evaporated onto a glassslide. It should be understood that this experimental arrangement, shownschematically in FIG. 4, is sensitive primarily to the electronicproperties of the microstructured layer, and not that of the substrate.

With continued reference to FIG. 4, the resistivity measurements can beperforated by applying a small DC current (e.g., 10 microamperers) fromcorner 2 to corner 1 while measuring a voltage from, corner 3 to corner4. Hall effect measurements can be performed by placing the sample in astrong magnetic field (e.g., a field of several thousand gauss) with themagnetic Held perpendicular to the silicon surface. In one experiment, asmall AC current (about 1-2 microamperes) was then applied from contact1 to contact 3 while measuring an induced voltage across contacts 2 and4. Control measurements were also performed on an n-doped(resistivity=8-12 ohm-m) unstructured silicon wafer. Table 1 below liststhe results of these measurements for microstructured layers formed inan n-doped silicon wafer having a resistivity in a range of about 8-12ohm-m at a number of different laser pulse fluences.

Average Sheet Sheet carrier influence Doping after resistance Rs densityn, Hall mobility μ Original doping (kJ/m²) structuring (Ohm) (cm⁻²)(cm²V⁻¹s⁻¹) n unstructured — 31931 1.78 × 10¹¹ 1101 n 4.0 n 4865 7.37 ×10¹² 174 n 6.0 n 4992 7.15 × 10¹² 175 n 8.0 n 6253 4.52 × 10¹² 221 n10.0 n 4554 8.83 × 10¹² 155

This data shows that microstructured layer exhibits a higher sheetcarrier density than the original substrate, but a lower carriermobility—presumably as a result of the disorder in the microstructuredlayer.

Referring again to the flow chart 10 of FIG. 1, subsequent toirradiation of the silicon surface with a plurality of laser pulses(step 10 a), the substrate can annealed (step 10 b) at a sufficientlyelevated temperature for a selected time duration so as to cause anincrease in the charge carrier density in the microstructured layer,e.g., by a factor in a range of about 10 percent to about 200 percent.For example, the substrate can be annealed at a temperature in a rangeof about 500 K to about 1100 K, and more preferably, in a range of about500 K to about 900 K, for a selected time duration, e.g., a timeduration in a range of about a few minutes to about a few hours (e.g.,one-half hour).

The annealing step can be performed by employing a variety of differenttechniques. For example, the substrate can be placed in a thermal ovento be exposed to an elevated temperature for a selected duration.Alternatively, the substrate can be exposed to laser radiation, e.g.,radiation having a wavelength in a range of about 200 to about 1200 nm,to cause its heating to a desired temperature. Further, the annealingstep can be performed, for example, in an inert atmosphere (e.g., anargon atmosphere) or in a vacuum (a low-pressure environment).Alternatively, the annealing step can be applied simultaneously with theirradiation step. That is, the irradiation of the silicon substrate withthe laser pulses can be performed while the substrate is maintained atan elevated temperature.

Without being limited to any theory, the annealing step is designed tocause a rearrangement of the atomic bonds within the metastablemicrostructured layer to enhance the density of chargecarriers—electrons—within that layer. The term “charge carrier density”is known to those having ordinary skill in the art. To the extent thatany further explanation may be required, it refers to density of thosecharged particles, e.g., electrons, that are primarily responsible forcurrent conduction, e.g., electrons in the conduction band states or inshallow impurity states below the conduction band. Such charged carriersare also herein referred to as free electrons or holes.

In other words, the annealing temperature and duration are selected tocause structural changes in the microstructured layer that result inincreasing the carrier density in that layer while substantiallymaintaining responsivity of the layer for generating an electricalcurrent when exposed to radiation within a selected wavelength range ata given applied back bias voltage, as discussed in more detail below.The structural changes caused in the microstructured sulfur-rich layerby an annealing step according to the teachings of the invention can beappreciated by considering that in many embodiments in which a p-dopedsilicon substrate is exposed to a plurality of laser pulses to generatea microstructured surface layer therein, holes, rather than electrons,constitute the dominant charge carriers even alter incorporation ofabout 1 atom percent sulfur having two possible donor electrons peratom. However, after annealing, electrons can constitute the dominantcharge carriers in the microstructured layer. Without being limited toany particular theory, such observations suggest that sulfur in themicrostructured layer, before annealing, is incorporated in silicon suchthat its donor electrons do not substantially contribute to conduction(the donor electrons are likely locked up in a trap, or perhaps thecoordination number of the implanted sulfur is larger than four). Anannealing step according to the teachings of the invention causes atomicbond rearrangements in the microstructured layer so as to free up donorelectrons for contributing to electrical conduction. In fact, in case ofp-doped silicon substrates, the number of donor electrons releasedduring annealing can be sufficiently large to eliminate the holes as thedominant charge carriers and turn the microstructured layer into ann-doped layer having a sheet carrier concentration on the order of about10¹⁴ cm⁻².

A microstructured silicon water formed in accordance with the teachingsof the invention exhibits absorption of incident electromagneticradiation, particularly radiation having wavelengths in a range of about250 nm to about 2500 nm. By way of example, FIG. 5 presents absorptiondata for a microstructured silicon wafer (an n-Si (111) wafer having athickness of about 260 microns and a resistivity in a range of about8-12 ohm-m), each of which was generated by exposing its surface to 100femtosecond laser pulses (2 to 500 shots per location) having a centralwavelength of 800 nm and a fluence of about 8 kJ/m². This exemplaryabsorption data was recorded in a manner known in the art, prior toannealing the wafer, by employing an spectrophotometer equipped with anintegrating sphere. The absorption data for an unstructured siliconwafer is also provided for comparative purposes.

This exemplary data indicates that the microstructured wafers exhibit anenhanced absorption of incident electromagnetic radiation, relative tounstructured silicon, across the entire recorded wavelength range, andparticularly for wavelengths above about 1050 am, which corresponds tothe band-gap energy of crystalline silicon (1.05 eV). In theunstructured silicon, light photons having wavelengths longer than awavelength corresponding to the band-gap energy do not containsufficient energy to promote an electron from the valence band to theconduction band, leading to a drastic drop in absorption.

In contrast, the silicon wafers microstructured in accordance with theteachings of the invention exhibit significant absorption not only forwavelengths below the band-gap but also for those above it. In fact, theexemplary data, of FIG. 5 shows that silicon waters structured byexposure to 20 pulses per location or higher (e.g., 500 pulses in thisexemplary data set) exhibit near-unity, featureless absorption ofwavelengths from about 0.4 micron to about 1 micron, a small decrease inabsorption for wavelengths around about 1.05 micron (a wavelengthcorresponding to the band edge of unstructured silicon), and strongfeatureless absorption from about 1.05 microns to about 2.5 microns. Itshould be understood that this data is presented only for illustrativepurposes, and is not necessarily intended to present the optimalabsorption of a silicon wafer structured in accordance with theteachings of the invention.

As noted above, in many embodiments of the invention, a microstructuredwafer is annealed at a temperature and for a duration designed toenhance the charge carrier concentration in its microstructured layer.Such annealing can be performed at a variety of different temperatures.By way of example, FIG. 6 schematically illustrates electromagneticradiation absorptance data for a plurality of microstructured siliconwafers generated by utilizing 500 laser pulses per surface location (100fs pulses at a central wavelength of 800 nm and a fluence of 8 kJ/m2) inthe presence of 0.67 bar of SF₆ relative to that of unstructuredsilicon. Each structured wafer was annealed at a selected temperature ina range of about 575 K to about 875 K for about 30 minutes. Thisexemplary data shows that annealing at temperatures below about 575 Khas little effect on the observed absorptance. On the other hand, suchannealing performed at a temperature between about 575 K and 875 K doesnot substantially affect absorptance above the bandgap, but lowers thebelow band-gap absorptance—the higher the annealing temperature, thegreater is the reduction in absorptance. It should, however, beunderstood that this data is only exemplary, and different results maybe obtained for microstructured wafers generated by employing differentparameters (e.g., difference fluence or laser shot numbers).

A microstructured silicon wafer according to the teachings of theinvention can be generated not only by utilizing femtosecond pulses, butalso other pulse widths (e.g., nanosecond and picosecond pulse widths).By way of example and only for illustrative purposes, FIG. 7 illustratesexemplary absorptance graphs as a function of incident, radiationwavelength for femtosecond-laser-formed and nanosecond-laser-formedmicro structured silicon surfaces, before and after annealing at about875 K for about 30 minutes, as well as that of an unstructured siliconwafer. The femtosecond-formed microstructures were generated byemploying 100 fs laser pulses (500 shots per location) having a centralwavelength of about 800 nm and a fluence of about 8 kJ/m². Thenanosecond-formed microstructures were generated by employing 30 nslaser pulses (about 1500 shots per location) having a central wavelengthof 248 nm and a fluence of about 30 kJ/m². It should be understood thatthis data is presented only for illustrative purposes and is notintended to necessarily indicate absorptance exhibited by amicrostructured silicon formed in accordance with the teachings of theinvention under all circumstances.

In some embodiments of the invention, microstructuring of a siliconwafer can be accomplished in the presence of a background fluid ratherthan SF₆. For example, H₂S, Cl₂, N₂ or air can be utilized. By way ofexample, FIGS. 8A and 8B compare absorptance of some exemplary prototypemicrostructured silicon wafers generated by employing femtosecond laserpulses (100 fs, 800 nm, 8 kJ/m² and 500 shots per location) in thepresence of various fluids—without an annealing step—with that ofunstructured silicon. The vacuum data corresponds to microstructuringthe wafer in a chamber evacuated to a base pressure that is less thanabout 10⁻⁶ bar. While all microstructured samples exhibit an enhancedabsorption relative to unstructured silicon, this exemplary data showsthe microstructuring in SF₆ and H₂S brings about the most dramaticchange in absorptance relative to unstructured silicon. In particular,samples structured with SF₆ exhibit near-unity, substantiallyfeatureless absorptance across the entire measured spectrum.

A variety of background fluid pressures can be employed in manyembodiments of the invention for generating microstructured siliconwafers. By way of example, FIG. 9 shows absorptance data for a number ofexemplary prototype microstructured silicon wafers generated in SF₆ (100fs pulses, 800 nm, 500 shots per location) at different ambientpressures, as well as a wafer microstructured in vacuum (e.g., apressure of less than 10⁻⁶). The greatest rate of increase is observedat low pressures. In this exemplary data set, at pressures above about27 mbar, the below band-gap absorptance does not show much variationwith wavelength, while at lower pressures, it decreases with increasingwavelength.

The microstructured silicon wafers generated in accordance with theteachings of the invention, particularly those subjected to an annealingstep at an elevated temperature in a range of about 500 K to about 900K, not only exhibit enhanced absorption of electromagnetic radiationover a wide range (e.g., from ultraviolet wavelengths to near infrared),but they also exhibit voltage-versos-current profiles that arecharacteristic of diodes. As discussed in more detail below, theseproperties allow the use of such microstructured silicon wafers infabricating photodetectors that exhibit enhanced responsivity relativeto conventional silicon photodetectors.

More specifically, a microstructured silicon layer formed in accordancewith the teachings of the invention, particularly one formed by exposureof a wafer surface to laser pulses followed by, or simultaneous with, anannealing step designed to release electrons in the layer structured bythe pulses, forms a diode junction, e.g., a p-n junction, with theunderlying silicon substrate. It should be understood that a diodejunction as used herein can also refer a junction formed between twon-doped portions having different free electron densities. As shownschematically in FIG. 10, a p-n junction can be formed by bringing asemiconductor material having acceptor impurity atoms (p-doped) intocontact with a semiconductor material having donor impurity atoms(n-doped). The difference in charge carrier concentration between thep-doped and the n-doped portions generates a diffusion potential acrosstire junction—electrons and holes begin to diffuse across the junctionto portions having a respective lower concentration. As they cross tothe other side of the junction, they recombine with their oppositelycharged electrical counterparts in a region very close to the junction.This migration of charge carries across the junction causes formation ofspace charges (immobile ions) with opposite polarities on the two sidesof junction. As the number of space charges increases (positivelycharged for the n-type side and negatively changed for the p-type side),a potential difference builds up across the junction, which results inan electric field in a direction opposite that of the diffusionpotential.

Upon reaching thermal equilibrium, a thin insulating layer is present,near the junction in which charge carriers have been depleted byrecombination (depletion region). A strong electric field is present inthe depletion region and a large electric potential exists across thisregion. The junction can be reverse-biased by applying an electric Heldacross it that reinforces this internal electric field. Such areverse-bias voltage expands the insulating depletion region andenhances both the internal electric field and the potential differenceacross the junction. The junction can also be forward-biased by applyingan electric field that is opposite to the internal field across it. Sucha forward-bias voltage shrinks the depletion region, thereby diminishingthe internal electric field and the potential difference across thejunction. When forward-biased, a current flows across the junction withelectrons injected into the n-doped portion, and leaving from thep-doped portion. In other words, the p-n junction, can act as arectifier by allowing current flow in only one direction. This behaviorgives rise to a current-voltage (I-V) curve characteristic of a diode.

As discussed above, a silicon water in which a microstructured layer isformed according to the teachings of the invention can exhibit, an I-Vcurve characteristic of that obtained by a diode junction. Without beinglimited to any theory, such a current-voltage behavior can be understoodby considering that the wafer includes two adjacent, yet distinct,layers: a surface layer in which a concentration of an electron-donatingconstituent is incorporated and an underlying substantially undisturbedsilicon substrate layer. These two layers exhibit different crystallinestructures, chemical compositions, and charge carrier dopingconcentrations. In addition, the microstructured layer has uniqueoptical properties including high absorptance of incidentelectromagnetic radiation from the near ultraviolet into thenear-infrared wavelengths. In one aspect, the invention employs suchmicrostructured silicon wafers for fabricating photodiodes, as discussedin more detail below.

By way of example, FIG. 11A schematically illustrates a photodetector 46according to one embodiment of the invention that includes amicrostructured silicon wafer 48 having a microstructured surface layer50 that is formed in a silicon substrate 51 by irradiating a pluralityof locations on a substrate's surface with short laser pulses (e.g.,pulse widths in a range of about tens of femtoseconds to tens ofnanoseconds) while exposing the surface to a substance having anelectron-donating constituent (e.g., SF₆). The laser-irradiated wafer isthen annealed under conditions (e.g., at an elevated temperature and fora selected duration) designed to enhance concentration of electrons inthe microstructured surface layer (e.g., annealing at a temperature in arange of about 500 K to about 900 K for a duration of a few minutes to afew hours).

The exemplary photodetector 46, functioning as a photodiode, alsoincludes a plurality of metallic electrical contacts 52, which have alinger grid geometry, that are disposed on the microstructured surfacelayer. In addition, an electrical contact 54, in the form, of a metalliccoating layer, is disposed on the back surface of the silicon substrate(i.e., the undisturbed silicon surface opposite the microstructuredsurface) that substantially covers the entire surface. In this exemplaryembodiment, a chromium/gold alloy (Cr/Au) is employed for fabricatingthe electrical contact. However, those having ordinary skill in the artwill appreciate that the contacts can be formed of other metals, aswell.

The metallic contacts 52 and 54 allow applying a bias voltage, e.g., areverse bias voltage, across the wafer by employing, for example, avoltage source 56. The reverse bias voltage can reinforce the internalelectrical field in the depletion layer of a diode junction formedbetween the microstructured layer 50 and the underlying siliconsubstrate 51. As discussed in detail below, the photodetector 46 canoperate with small bias voltages, e.g., voltages in a range of about 0.1V to about 15 V, with a responsivity (i.e., photocurrent generated perunit of power of incident light, (amperes/watts)) that is considerablybetter than that typically exhibited by a conventional siliconphotodetector. These attributes allow incorporating siliconphotodetectors formed according to the teachings of the invention, suchas the exemplary photodetector 46, into integrated siliconphoto-electronic circuits as high sensitivity detectors.

In operation, radiation (i.e., photons) incident on the microstructuredsurface layer of the photodiode's silicon wafer is absorbed (when theradiation wavelength is in the operating range of the photodetector),e.g., by valence electrons, in the microstructured layer. The absorptionof the incident radiation causes generation of electron-hole pairs. Theelectric field within the depletion region causes separation of thegenerated electron-hole pairs, especially those created at or inproximity of the diode junction, which is formed between themicrostructured silicon layer and the underlying silicon substrate. Theseparated electron-hole pairs provide a photocurrent that isproportional to the number of incident photons (those skilled in the artappreciate that such a photocurrent is added to a generation current anda recombination current that can be inherently present in a diodejunction). Hence, incident radiation can be detected, and quantified, bymeasuring the induced photocurrent.

A photodetector formed according to the teachings of the invention notonly exhibits absorptance across a wide range of wavelengths (theabsorptance is generally commensurate with that exhibited by thephotodetector's microstructured silicon wafer), but it also exhibits adiodic I-V behavior. By way of example and further elucidation of thesalient features of a photodetector of the invention, FIG. 12illustrates a plurality of graphs, each representing current as afunction of bias voltage for a plurality of microstructured siliconsamples prepared under different conditions in an n-doped siliconsubstrate for use as the active component of photodetectors. All sampleswere irradiated with a plurality of femtosecond laser pulses (800 nm,100 fs, fluence of 4 kJ/m²) in the presence of SF₆, but were subjectedto different annealing conditions. The graph A represents thecurrent-voltage curve associated with a microstructured sample that wasnot annealed. This curve is substantially linear, indicating that thejunction between the microstructured layer and the underlying siliconsubstrate in this sample is not diodic, but is merely resistive. Inother words, such a wafer is not optimal for fabricating aphotodetector.

In contrast, graphs B, C, and D illustrate current-versus-voltagebehavior of those microstructured silicon samples that were subjected toannealing at different temperatures (the annealing duration was about 30minutes in all cases). This exemplary data indicates that with anincrease in the annealing temperature, the measured I-V curve approachesone expected from a diode. For example, at an annealing temperature ofabout 825 K, the I-V curve is diodic, characterized by an exponentiallyincreasing current for a forward bias and a smaller, less rapidlyincreasing current for a reverse bias. The I-V curve corresponding to anannealing temperature of about 1075 K is also diodic. However, annealinga microstructured silicon wafer for a significant duration at a veryelevated temperature can lead to a degradation of the below-hand gapabsorptance, and hence a degradation in the performance of aphotodetector utilizing such a microstructured wafer in thenear-infrared. In this exemplary data set, the substrate annealed at 825K exhibit an optimal I-V characteristic. It should, however, beunderstood that such very elevated annealing temperatures may beemployed so long as the annealing duration is sufficiently short. Ingeneral, the annealing of a microstructured silicon wafer to be employedin a photodetector of the invention should be carefully designed (i.e.,annealing temperature and duration) to cause release of electrons in themicrostructured surface layer without significantly degrading thewafer's absorptance of and responsivity to incident radiation.

A photodetector of the invention, such as the above exemplaryphotodetector 46, exhibits a responsivity that can be superior across awavelength range of about 250 nm to about 10 microns to that of aconventional silicon photodiode. By way of example and only forillustration and corroboration purposes, FIG. 13 presents experimentalgraphs representing responsivity of exemplary photodetectors formed inaccordance with the teachings of the invention, over a selectedwavelength range, in comparison with a conventional photodiode and thosefabricated by employing microstructured silicon, but with no annealingor annealing at too high a temperature. In particular, graphs A and Bshow that photodetectors of the invention that employ silicon wafersmicrostructured by femtosecond laser pulses in the presence of SF₆ (100fs, 800 nm central wavelength and a fluence of 4 kJ/m²) followed byannealing at 825 K or 725 K, exhibit a significantly enhancedresponsivity relative to that of a conventional commercially availablesilicon photodiode. In particular, these photodetectors exhibit aresponsivity that is more than an order of magnitude greater that thatof the conventional photodiode in a wavelength range of about 250 nm toabout 1050 nm. Although the responsivity shows a decrease forwavelengths longer than the band-gap edge (i.e., wavelengths longer thanabout 1050 nm), nonetheless, it remains significantly higher than thatexhibited by a conventional silicon detector. In fact, conventionalsilicon detectors typically do not operate at useful levels ofresponsivity at wavelengths that are longer than the hand-gap edge. Forexample, in this exemplary illustration, the responsivity of thecommercial photodiode drops steeply to about 10⁻⁶ A/W for wavelengthslonger than about 1050 nm

Further, the responsitivity data for wafers formed according to theteachings of the invention (e.g., graphs A and B) were obtained byapplying small bias voltages (e.g., a bias voltage of about −0.5 voltsin this exemplary embodiment). In contrast, conventional siliconphotodetectors exhibit much lower responsivities, or require much higherbias voltages (e.g., 100 to 1000 volts) and/or have more complexstructures. This indicates that while photodetectors formed inaccordance with the teachings of the invention can be readilyincorporated in integrated silicon circuitry, the incorporation ofconventional silicon detectors, which require large bias voltages, intointegrated silicon circuits can be impractical.

The data depicted in FIG. 13 also shows that proper annealing of asilicon wafer irradiated by a plurality of short laser pulses, asdescribed above, can considerably enhance the responsivity of aphotodetector that employs that wafer. In particular, a photodetectorfabricated by utilizing a wafer that, though exposed to laser pulses,was not annealed, exhibits a much degraded responsivity relative tothose that were subjected to an annealing step at appropriatetemperatures, e.g., at 725 K or 825 K. This exemplary data also showsthat a photodetector that incorporates a microstructured wafer annealed,subsequent to irradiation by laser pulses, at a temperature of 1075 forthirty minutes exhibits a much degraded responsitivity relative to thoseannealed at 725 K or 825 K.

Hence, one aspect of the invention relates to annealing a silicon wafer,subsequent to—or simultaneous with—its irradiation by a plurality ofshort pulses in the presence of a substance having an electron-donatingconstituent, under conditions that would release electrons in themicrostructured layer while preserving the responsivity of the wafer fordetecting electromagnetic radiation. For example, in some embodiments ofthe invention, the annealing temperature is selected to be in range ofabout 500 K to about 1100 K, and more preferably in a range of about 700K to about 900 K, and the annealing duration is selected to be in arange of about 1 minute to about several hours, and more preferably in arange of about 5 minutes to about 30 minutes. Alternatively, higherannealing temperatures can be employed so long as the annealing durationis properly selected to avoid degrading the responsivity of theresultant annealed wafer. For example, although the above data indicatesthat annealing the wafer at temperature of about 1075 K for about 30minutes may not be desirable, this annealing temperature with a lowerannealing duration can be suitable for practicing the invention incertain applications. To certain extent, annealing temperature andduration are inter-related. For example, shorter annealing time at ahigher temperature can be employed in certain applications.

In other words, it is not simply annealing at an elevated temperaturethat is required. Rather, the annealing temperature and its durationneed to be properly selected to ensure that a photodetector exhibitingenhanced responsivity over a wide wavelength range can be fabricated byutilizing the annealed wafer. It has been discovered that annealing asilicon wafer subsequent to, or simultaneous with, its exposure to shortlaser pulses in presence of a substance having an electron-donatingconstituent at a temperature that, is too low can lead to noisyphotodiode and annealing at too high a temperature and duration canlower the wafer's response to both visible and infrared wavelengths.More specifically, the annealing temperature and duration are preferablyselected to enhance the concentration of carrier electrons in themicrostructured layer—formed by laser irradiation in presence of asubstance having an electron-donating constituent—by a factor in a rangeof about 10% to about 200% while ensuring that the resultant annealedwafer exhibits a responsivity greater than, about 20 A/W at least at onewavelength, and preferably over the entire span, of a wavelength rangeof about 250 nm to about 1050 nm, and a responsivity greater than about0.1 A/W at least at one wavelength, and preferably over the entire span,of a wavelength range of about 1050 nm to about 3500 nm. By way ofexample, in some embodiments of the invention, the annealing temperatureis preferably selected to be in a range of about 700 to about 900 K.

The choice of the fluence of laser pulses irradiating a silicon wafer togenerate a microstructured layer therein can also affect the performancecharacteristics of a photodetector that incorporates the microstructuredwafer for detecting radiation. By way of example, FIG. 14 presentsexemplary data corresponding to current-voltage characteristics of aplurality of silicon wafers irradiated with a plurality of femtosecondlaser pulses (100 fs, 800 nm central wavelength, 200 pulses perlocation) in the presence of SF₆ followed by annealing at 825 K forabout 30 minutes. The laser pulse fluences, however, differed from onewafer to another. More particularly, the following laser fluences wereemployed: 4 kJ/m², 6 kJ/m², 8 kJ/m², and 10 kJ/m². This exemplary dataindicates that increasing the fluence decreases the dark current for agiven reverse bias voltage.

FIG. 15A illustrates the responsivity of a plurality of photodetectorsincorporating the above silicon wafers that were microstructured withdifferent laser pulse fluences (the wafers were annealed at 825 K for 30minutes). This exemplary responsivity data indicates that, in thissample set, the photodetector's responsivity decreases, and thewavelength of peak response shifts to longer wavelengths, withincreasing fluence (the sample made with a fluence of 4 kJ/m² exhibitsthe highest response). By way of further illustration, FIG. 15B presentsresponsivity data for photodetectors made with microstructured siliconwafers that were generated through irradiation with femtosecond pulses(100 fs, 800 nm central wavelength, 200 pulses per location) in thepresence of SF₆ followed by annealing at 1075 K for about 30 minutes.The laser pulse fluences, however, varied from one sample to the next (4kJ/m², 6 kJ/m², 8 kJ/m², and 10 kJ/m²). A comparison of this data withthe data presented in FIG. 15A shows that the responsivity exhibited bythe samples subjected to the higher annealing temperature of 1075 ismuch lower that that of the samples annealed at 825 K. However, thesamples annealed at the higher temperature exhibit much less variationin responsivity as a function of fluence. Further, the samples annealedat 1075 K—though exhibiting a lower responsivity—are less noisy and havea higher quantum efficiency.

It should be understood that the above data is presented only forillustration purposes, and is not intended to necessarily indicate anoptimal fluence under all conditions. For example, the above exemplarysamples were made with an average laser shot number of about 200 pulses,and may exhibit improved characteristics with a greater shot number. Ingeneral, in various embodiments of the invention, the laser pulsefluence is selected to be greater than about 3 kJ/m². More preferably,the fluence is chosen to be in a range of about 3 kJ/m² to about 10kJ/m², or a range of about 3 kJ/m² to about 8 kJ/m².

In many embodiments of the invention an n-doped silicon substrate isemployed for forming a microstructured wafer, as described above.Alternatively, a p-doped silicon water can be utilized. By way ofexample, FIG. 16A presents graphs comparing the diodic characteristicsof a microstructured wafer generated according to the teachings of theinvention, such as those described above, employing a p-doped siliconsubstrate (p-Si(100), 350 micron thick, resistivity (ρ) greater than 1ohm-cm) with one fabricated utilizing an n-doped silicon substrate. Inboth cases, a laser pulse fluence of about 6 kJ/m², an annealingtemperature of about 825 K, and an annealing duration of about 30minutes were utilized. Similar data is presented in FIG. 16B forp-dopend and n-doped samples annealed at 1075 K for about 30 minutes.

This exemplary data shows that the p-doped samples annealed at 825 Kexhibit a higher forward bias current than equivalently generatedn-doped samples. The dark current for back biases also increases morerapidly for p-doped samples. Upon annealing at 1075 K, the p-dopedsamples exhibit very good diodic properties with the back biasdecreasing to substantially the same level as that in equivalentlygenerated, n-doped samples. Further, the forward bias current increasesmore rapidly, resulting in good rectification ratios. As noted above,annealing at higher temperatures can increase donor concentration fromsulfur doping. Such increase in the donor concentration can lead p-dopedsamples annealed at higher temperature to exhibit better rectification.

A silicon substrate's doping can also affect the responsivity of asilicon wafer formed by microstructuring that substrate in accordancewith the teachings of the invention. By way of example, FIG. 17A showsmeasured responsivity data for two n-doped microstructured wafers andtwo p-doped microstructured wafers, both of which were annealed at 825 Kfor about 30 minutes. This exemplary data indicates that theresponsivity of n-doped substrates is higher over the measured spectralrange. However, FIG. 17B shows that the responsivities ofmicrostructured wafers formed in p-doped and n-doped substrates, vialaser irradiation and annealing at a higher temperature of about 1075 K,are nearly identical.

In general, the above illustrative data indicates that microstructuredp-doped samples have a lower response but exhibit a better quantumefficiency. It should be understood that the above data is presentedonly for illustrative purposes and is not intended to limit the scope ofthe invention. In general, both p-doped and n-doped substrates can beemployed in the practice of the invention, and the resultant diodiccharacteristics and responsivity of microstructured wafers generated byutilizing such substrates can be different than those depicted abovebased on selection of other parameters, such as laser fluence, number ofpulses, etc.

In addition to responsivity, another parameter of interest associatedwith performance of a photodetector can be its quantum efficiency. Theterm ‘quantum efficiency’ is known in the art, and is generally definedas the number of electron-hole pairs generated per incident photon at azero applied voltage. In some cases, a microstructured sample thatexhibits a high level of responsivity may not show a correspondinglyhigh level of quantum efficiency. For example, in the above exemplaryprototype microstructured silicon wafers, samples fabricated byemploying n-doped substrates, laser pulses having a fluence of 4 kJ/m²,and an annealing temperature of about 825 K exhibit the highestresponsivity, but not the highest quantum efficiency. In fact, samplesfabricated by employing similar conditions but a higher annealingtemperature of 1075 K show higher quantum efficiencies, though lowerresponsivities. Similarly, the measured responsivities of samples madeon p-doped substrates and annealed at 825 K or 1075 K are lower, buttheir quantum efficiencies are higher. Such variations of responsivityand quantum efficiency are preferably considered when selecting variousparameters in practicing the teachings of the invention for fabricatinga silicon photodetector.

The responsivity of a photodetector fabricated in accordance with theteachings of the invention can also vary as a function of a back biasapplied thereto. Typically, the responsivity increases as the appliedhack bias increases. By of example, FIG. 18 exhibits the responsivity ofa silicon wafer microstructured in accordance with the teachings of theinvention through irradiation by a plurality of femtosecond pulses at afluence of 4 kJ/m² followed by annealing at 825 K for about 30 minutes,as a function of an applied back bias voltage. The sample wasilluminated with a white light source with an approximate power of 50microwatts. The responsivity, and hence gain, increases quickly up to abias voltage of −5 volts and more slowly up to −15 volts. At voltagesbeyond −15 volts, the responsivity begins to fall. In some cases, theresponsivity of a photodetector that exhibits a large gain may not besufficiently linear for certain applications. Hence, in selecting aproper back voltage, in addition to responsivity, other factors, such asthe linearity of the response should be considered, in addition, inapplications in which the photodetectors are incorporated in anintegrated silicon circuit, the bias voltage should be compatible withthe requirements of that circuit.

A photodetector formed according to the teachings of the invention canoperate within a wide range of temperatures. In some cases, it may beadvantageous to cool the photodetector to decrease its average noiselevel. By way of example, FIG. 19A presents current-voltage curvescorresponding to an n-doped microstructured silicon sample made withfemtosecond pulses at a fluence of about 4 kJ/m² and annealed at 825 Kat different operating temperatures. The measured current in both theback bias and forward bias conditions decreases with decreasing thetemperature. At very low temperatures (below 100 K), conduction becomesvery low for both forward and back biases, FIG. 19B shows theresponsivity of such a microstructured wafer to incident radiationhaving a wavelength of 1064 nm (a wavelength close to the band gap ofsilicon at room temperature) at a back bias of −0.5 V as a function ofoperating temperature. The measured responsivity drops with temperature.This behavior may be, however, different if the wavelength of theilluminating light is far from the silicon band gap. A graph of the darkcurrent at a bias voltage of −0.5 V as a function of temperature,however, shows that the noise decreases much more rapidly that theresponsivity.

In sum, photodiodes formed according to the teachings of the inventioncan exhibit responsivities in a wide region of the electromagneticspectrum (e.g., wavelengths in a range of about 250 nm to about 1100 nm)that are nearly two orders of magnitude higher than those exhibited byconventional commercial silicon photodiodes. Further, responsivities atlonger wavelengths, e.g., as long as 1600 nm, can be nearly five ordersof magnitude greater than those exhibited by conventional siliconphotodiodes in this range. In some of the above prototype samplesfabricated according to the teachings of the invention for illustrationpurposes, the carrier mobility in the surface layer was measured to beabout 101 cm² V⁻¹ s⁻¹; the response times included, a rise time of about10 ns and a fall time of about 30 ns; and a diode capacitance of about63.9 nF at a back bias of −0.5 V was measured, A photodiode of theinvention can exhibit a large gain at very small applied bias voltages,e.g., a gain greater than 1000 for an applied bias voltage of −0.5 V.

A silicon photodetector according to the teachings of the invention,such as the above exemplary photodetector 46 (FIG. 11A), can befabricated by employing a variety of techniques. By way of example, inone exemplary method of fabricating such a photodetector, a selectedportion of a silicon wafer, e.g., a 5×5 mm² area, is microstructured viaexposure to short laser pulses in the presence of a suitable material,e.g., SF₆, in a manner described above. The wafer is then dipped in asolution of HF (e.g., a 5% solution) in order to remove any native oxidelayer before forming electric contacts with the microstructured layer.Subsequently, a mask is deposited over the surface of themicrostructured layer having exposed portions which correspond to adesired pattern of a conductive electrode to be formed on that surface.A selected metal, e.g., a Cr/Au alloy, is then evaporated over themasked surface, and the mask is removed to leave behind a patternedconductive layer, such as the conductive layer 52 shown in FIG. 11A, anduncoated portions suitable for receiving radiation. Further, aconductive layer can be formed on a back surface of the wafer, anunstructured surface opposed to the microstructured layer, such as thelayer 54 shown in FIG. 11A. In many embodiments, the entire back surfaceis coated with the metal. The wafer can be then be coupled to a voltagesource, such as the source 56 shown in FIG. 11A, that can apply a biasvoltage thereto and optionally other electronic elements, e.g.,capacitors, in a manner known in the art. Further, a the water and itsassociated circuitry can be placed in an appropriate housing byemploying techniques known in the art.

The applications of microstructured silicon wafers formed according tothe teachings of the invention, such as those discussed above, are notlimited to photodetectors. For example, such silicon wafers can beemployed in fabricating solar cells. In some cases, the optimal valuesof various parameters, such as substrate doping, associated with themicrostructured wafer utilized in generating solar cells can bedifferent than those in fabricating silicon photodetectors. A solar cellcan be considered a p-n junction photodiode with high quantumefficiency. The conversion of photons into photocarriers is the physicalmechanism by which light is converted in a solar cell into electricity.However, there is a fundamental difference between a photodetector and asolar cell. A photodetector is generally operated in a “photoconductive”mode: a back bias is applied thereto and a current is measured. Incontrast, a solar cell is operated in a “photovoltaic” mode: if isoperated in an open circuit mode or attached to a load withoutapplication of a bias voltage.

In one embodiment, a microstructured layer is formed in a p-dopedcrystalline silicon substrate by initially irradiating a plurality oflocations on a surface thereof with femtosecond (e.g., 100 fs, and acentral wavelength of 800 nm) laser pulses having a fluence of about 4kJ/m² followed by annealing at 1075 K for about one hour. The wafer canthen be utilized in fabricating a solar cell. More specifically, metalcontacts, e.g., Cr/Au contacts, can be evaporated on the microstructurelayer's surface so as to coat selected portions thereof while leaving atleast a portion of the surface uncovered for receiving radiation, e.g.,a linger grid metallic pattern can be disposed on the surface. Inaddition, the back, surface of the substrate, the unstructured surfaceopposing the structured surface, can be coated, e.g., via evaporation,with a metallic coating. A load can be attached the wafer via electricalcoupling to the metallic layers on the both sides of the wafer.

FIG. 20 presents data comparing current-voltage (I-V) behavior of such awafer, formed in a high resistivity (resistivity greater than about 1ohm-m) silicon substrate, that can be utilized as a solar cell in thepresence and in the absence of illumination. And FIG. 21 presents asimilar comparative data for a wafer formed in low resistivity siliconsubstrate (resistivity in a range of about 0.01-0.1 ohm-m). The solarcell fabricated by employing the high resistivity substrate has an opencircuit voltage of 0.4 volts and a short circuit current of 13.4 mAwhile the solar cell fabricated by utilizing the low resistivitysubstrate has an open circuit voltage of 0.31 volts and a short circuitcurrent of 13.45 mA. It should be understood, that this data ispresented only for illustration purposes, and to show the possibility ofutilizing a wafer microstructured in accordance with the teachings ofthe invention in forming a solar cell. However, this data is notintended to indicate optimal performance of a solar cell generated inaccordance with the teachings of the invention. For example, thedeposition of electrical contacts on the substrate can be optimized toobtain better performance than that described above.

In another application, a silicon wafer microstructured in accordancewith the teachings of the invention can be utilized as a field emitter.In other words, an electric field can be applied to the microstructuredsurface to cause emission of electrons therefrom. To show thepossibility of utilizing a microstructured wafer for generating fieldemission current, a number of prototype exemplary microstructured waferswere fabricated and field emission currents generated by them weremeasured. The experimental step-up for performing these measurements isshown schematically in FIG. 22. A microstructured silicon sample 58 witha gold coating 60 evaporated on a hack surface is mounted to a siliconwafer 62 having a gold coating 64, which functions as an anode, and isseparated therefrom by a pair of mica spacers 66 and 68. The mountedsample is placed in a vacuum chamber, which is evacuated to a pressureof about 10⁻⁶ torr. A potential difference can be applied between themicrostructured sample 58 and the gold-coated anode. And emissioncurrent can be measured at each applied voltage, e.g., via a picoammeter70, a resistive elements 72 placed in series with the ammeter to protectit from unexpected surges in current.

FIG. 23 shows exemplary results of field emission currents measured forsilicon samples microstructured in SF₆. The typical figures of merit fora Held emitting surface are turn-on-field, defined as the electric field(bias voltage divided by the tip-to-anode spacing) for which a currentdensity of 0.01 microampere/mm² is observed, and threshold field,defined as the field at which a current density of 0.1 microamperes/mm²is produced. The turn-on field for a plurality of exemplary prototypesubstrates microstructured in SF₆ was measured as 1.3 V/micrometer, andthe threshold field was measured as 2.15 V/micrometer—excellent valuesthat are on par with those exhibited by nanotubes.

Those having ordinary skill in the art will appreciate that variousmodifications can be made to the above embodiments without departingfrom the scope of the invention. It should also be understood thatvarious exemplary data presented above are intended only forillustrating salient features of various aspects of the invention, andis not intended to limit the scope of the invention.

What is claimed is:
 1. A method of processing a semiconductor substrate,comprising depositing a solid charge-donating substance on a surface ofa semiconductor substrate, subsequently, irradiating said surface with aplurality of laser pulses having a pulse width in a range of about 50femtoseconds to about 50 picoseconds incorporating at least a portion ofsaid solid charge-donating substance into a surface layer of saidsemiconductor substrate at a concentration in a range of about 0.5 toabout 5 atom percent, wherein a diode junction is formed with anunderlying portion of the substrate.
 2. The method of claim 1, whereinthe solid charge-donating substance is a solid electron-donatingsubstance.
 3. The method of claim 2, wherein said solidelectron-donating substance comprises any of selenium and tellurium. 4.The method of claim 1, wherein said pulse width is in a range of about50 femtoseconds to about 500 femtoseconds.
 5. The method of claim 1,wherein said semiconductor substrate has an electrical resistivity in arange of about 0.001 ohm-m to about 10 ohm-m prior to said step ofirradiating.
 6. The method of claim 1, wherein said pulses have fluencein a range of about 1 kJ/m² to about 12 kJ/m².
 7. The method of claim 1,wherein said pulses have a central wavelength in range of about 200 nmto about 1200 nm.
 8. The method of claim 1, wherein said semiconductorsubstrate is a silicon substrate.
 9. The method of claim 1, wherein saidpulses are applied to the semiconductor substrate at a repetition ratein a range of about 1 kHz to about 1 MHz.